Integrated circuits are typically fabricated on wafers of a single-crystal semiconductor material. The wafers are sliced from a single crystal ingot grown in a Czochralski-type crystal growing machine, referred to as a "crystal puller." Typically, many crystal pullers are located within a single large room, called a "growing hall." A control console located by each crystal puller allows a crystal puller operator to control each crystal puller in accordance with instrument readings and observations through an observation window of conditions within the crystal puller.
To grow an ingot, a seed crystal is dipped into a crucible of molten semiconductor and then slowly retracted as the seed and the crucible are rotated in opposite directions. The semiconductor freezes onto the seed as it is retracted to produce a single crystal ingot. Crystals are sometimes grown in the presence of a strong magnetic field which, by controlling thermal convection in the molten semiconductor, produces an ingot having a more uniform amount of oxygen incorporated into the ingot from the crucible walls.
The growing process begins with loading into a crucible a "charge" of ultrapure polysilicon. A dopant is often added to the charge to change the electrical properties of the resultant crystal. A seed crystal having the desired crystal orientation is then secured within the crystal puller in a chuck attached to a cable that is used to raise the seed, and a charged crucible is loaded into the crystal puller. The section of the crystal puller in which the seed crystal and the charge are placed is called the "furnace tank." As the single-crystal ingot is grown, it is received into a "pull chamber" above the furnace tank. A "mechanical unit" below the furnace tank includes motors and other mechanical and electrical devices used in the crystal growing process.
Before melting the charge, the air in the furnace tank and pull chamber is evacuated and replaced with an inert gas, such as argon. The crucible is rotated as the charge within it is melted, typically using a resistant heater. The seed crystal is dipped slightly into the melted charge and slowly retracted to grow the ingot. After the ingot is grown and cooled, the pull chamber is opened and the ingot is removed.
Extremely small amounts of contamination in a crystal can have severe adverse effects on the characteristics of electronic circuits fabricated on the crystal. It is critical, therefore, that airborne contamination be minimized in the growing area. Although clean room techniques for reducing airborne particulate contamination are widely used in the semiconductor industry, it is difficult and expensive to maintain a high degree of cleanliness in a large growing hall. The energy cost of providing the required volume of filtered airflow is too great. Higher than optimum particulate levels have, therefore, been tolerated in growing halls, along with corresponding contamination of the grown crystals.
Cross-contamination problems are much worse during the installation of new equipment. Growing halls typically include an overhead bridge crane that is used to install new equipment and repair existing equipment. Use of such an overhead crane produces particulate contamination that settles down onto the active crystal pullers.
To reduce contamination of the crystals during the installation of new crystal pullers, one practice has been to install several machines at one time in a segregated area of the growing hall. Under these circumstances, the growing hall is not being continually contaminated and disrupted by the installation of individual machines.